1. Field of the Invention
The present invention relates generally to improvements in servo control methods for rotating disk data storage devices, and, more particularly, but not by way of limitation, to improvements in methods for effecting transitions between selected offsets of a servo head from a servo track on a dedicated servo surface on a disk of the device.
2. Brief Description of the Prior Art
In rotating disk data storage devices developed to store large amounts of data received from a computer, it is common practice to mount a plurality of hard disks, each having a magnetic coating on both sides, on a common spindle to be rotated together. A movable actuator adjacent the disk stack supports a plurality of data heads adjacent selected surfaces--data surfaces--formed by the magnetic coatings and the actuator is controlled by a servo control circuit to radially position the data heads so that the data heads will overlie, or follow, selected circular data tracks defined on the data surfaces.
In one type of device in widespread use, the servo control circuit receives position error signals from a servo head adjacent one surface--a dedicated servo surface--of one disk and positions the actuator to position all heads, data and servo alike, in unison to effect following of the data tracks by the data heads via following of servo tracks magnetically written on the dedicated servo surface. To this end, the servo head and the data heads are arranged along a line parallel to the rotation axis of the disk and the data and servo tracks are grouped into coaxial cylinders centered on such rotation axis. Thus, positioning of the servo head to follow a selected servo track will cause each data head to follow the data track on the surface adjacent the data head that is contained in the same cylinder as the selected servo track.
A difficulty that occurs in this type of device is that the alignment of the data and servo heads necessary for data track following to be accomplished by servo track following cannot be maintained within allowable tolerances for reading and, more particularly, writing a data track. The spacing between radially successive cylinders is often less than a thousandth of an inch so that, when the width of the heads and consequently the data tracks, is taken into account, control of the radial position of the data heads must be effected to a tolerance measured in microinches to prevent a data head from reading or writing to a data track adjacent the track that is being followed. Thus, a very small misalignment between a data head and the servo head can result, for example, in overwriting of one data track, to destroy the data thereon, during writing to an adjacent data track. Such misalignments can occur in a number of ways; for example, through differential thermal expansion of the disks or of arms on which the data heads and the servo head are mounted, by mechanical shock experienced by the device to cause small realignments of connections between the arms and the actuator, and so on.
A solution to this misalignment problem has been described in "Correction of Data Track Misregistration in Servo Controlled Disk Files", by A. Paton, IBM Technical Disclosure Bulletin, Volume 17, Number 6, pages 1781 through 1783, published in November 1974. As disclosed in this bulletin, the teachings of which are hereby incorporated by reference, servo tracks are written to the data surfaces to be read while the servo head follows a servo track on the dedicated servo surface to determine the offsets between the data tracks and the data heads during track following by the servo head and these offsets are saved and used to generate offset signals that are transmitted to the servo control circuit when data tracks are followed. These offset signals will cause the actuator to offset the servo head from a servo track by an amount that is just sufficient to correct for the misalignment between a selected data head and the servo head with the result that the data head follows the data track.
While this solution for correcting misalignment of the data heads and the servo head has worked well, it is an incomplete solution to the problems that arise from the misalignment. In particular, the misalignment also has a deleterious effect on the transfer rate of data to and from the disks of the device. The amount of data to be stored on or recovered from a disk will often exceed the amount that can be stored on one data track and, in such cases, the transfer rate can be maximized by storing the data on first one track of a given cylinder and then proceeding to a second track on the same cylinder. Thus, large adjustments of the actuator from one cylinder to the next that take large amounts of time are made only when a cylinder has been completely written or a new file is read or stored. For this approach to maximize transfer rate, it is important that the transition between track following by one data head to track following by another be made as quickly as possible. However, a change in the offset of the servo head from the servo track on the same cylinder as the two data tracks must be effected prior to commencing reading or writing by the second data head. In the past, this offset change requirement has limited data transfer rates by requiring that sufficient time be allowed after the offset signal has been changed to enable stable track following by the second data head to occur.